Well from a ppd/wtt standpoint a 2p/4p folding machine would be most cost effective since you do get bonus points nd can run bigadv. I hear many tales that 4p systems have power draw as low as ~500w. Even from a parts standpoint a 4p system will probably cost less than a 4gpu setup for the same ppd. Of course the cheapest way to do things is from used or leftover parts. If you ever plan on upgrading an older 4core cpu and motherboard you can keep your old parts and you have the basis of a smp folding system.